The controls used to be designed so that you had to find a position to take aim, and you couldn't move while aiming. Granted, there was a lock on, but if you wanted clean, one hit kills, you had to be stealthy. You couldn't just kill all the guards when you were caught like it's a third person shooter. That seems to be the style of the most recent MGS games. Now, instead of being careful to not get caught while taking out guards, you can just kill everyone with your abundance of lethal weapons, and third person shooter mechanics.
MGS4 also had far too many lethal weapons, and wasn't even very stealthy.